At the present time, most single-family homes and small multiple-family homes that are not connected to a central sewer system employ a septic tank system for the disposal of biodegradable wastes, particularly sewage. These tanks are generally of very low efficiency in processing these wastes, and, as a result, often tend to pollute the environment. Because of the well known pollution problems that are generated by septic tank systems, regulatory authorities in many area have banned the use of such tanks, and it is expected that such bans will become more prevalent in the future. Accordingly, there is a need for a high-efficiency unit for the treatment of biodegradable wastes which is compact, inexpensive, and can be used to provide such treatment for single-family dwellings and small multiple-family dwellings.
Generally, such units provide a means for treating a liquor containing biodegradable wastes which comprises delivering the liquor to a quiescent settling zone, in which large particles are permitted to settle to the bottom where they are subjected to digestion by anaerobic microorganisms. The liquor is delivered from the settling zone to an aeration zone, where it is aerated, and then delivered downwardly through a column of submerged fixed media. The fixed media is composed of a multiplicity of elements which are freely and randomly stacked on one another to provide interstitial area between them. These elements have a high surface area to volume ratio, so that aerobic microorganisms can grow on their surface to treat biodegradable wastes contained in the liquor. As a result of their random stacking and high surface area, the elements promote homogeneous contact of the liquor with the fixed media. The liquor is received at the bottom of the fixed media and recirculated through the aeration zone and back through the fixed media in order to provide multiple passes through the media for thorough treatment. After such treatment, the effluent that has passed through the fixed media is withdrawn from the unit.
Examples of such units are disclosed in U.S. Pat. Nos. 3,966,599, 3,966,608 and 3,972,965, which patents are commonly assigned to the same assignee as the present invention.
These small aerobic sewage treatment units achieve very good results in treating household and small commercial waste waters in reduction of pollutants discharged to the environment. However, nutrients, such as phosphorous and nitrogen, may pass through these units at concentrations high enough to be considered detrimental to the environment or the ground water. In sewages, nitrogen is typically present in the trinegative forms of ammonia and organically bound nitrogen. The excess nitrogen over the quantity required by the biological treatment process is normally oxidized to nitrate-nitrogen (NO.sub.3 --N) and concentrations in the effluents may range from 10 to 30 mg/l. Many regulatory agencies are requiring that total nitrogen and specifically nitrate-nitrogen levels be reduced to lower levels.
The known biological process of denitrification involves the conversion of the nitrate-nitrogen to a gaseous nitrogen species. The gaseous product is primarily nitrogen gas but also may be nitrous oxide or nitric oxide. Gaseous nitrogen is relatively unavailable for biological growth thus denitrification converts nitrogen which may be in an objectionable form to one which has no significant effect on environmental quality. A relatively broad range of bacteria can accomplish denitrification. These organisms use either nitrate or oxygen as the terminal electron acceptors while oxidizing organic matter. Nitrate reduction is accomplished by a nitrate dissimilation process whereby nitrate or nitrite replaces oxygen in the respiratory process of the organism under anoxic conditions. This process is known as anoxic denitrification.
The heretofore small aerobic sewage treatment units have tended to have uneven BOD loading in the aeration section due to uneven influent flow rates into the unit. As a consequence, the effluent quality has been inconsistent in situations where the unit cycles between periods of light and heavy loading.
Accordingly, there is a need for an improved method and apparatus for treatment of liquor containing biodegradable waste material, such as sewage, that reduces the total nitrogen discharged in the effluent and that is able to maintain consistent effluent quality during cycles of uneven influent loading.